1. Field of the Invention
The present invention relates to an apparatus and method for improving the reliability and longevity of optical properties of an optical system by preventing, suppressing, or improving degradation of optical properties of an optical system lying in output light or along a light path of said output light, wherein said optical system is provided within a near vacuum zone where organic components may be decomposed, said degradation is caused by carbon deposited or accumulated upon light irradiation surfaces, light reflection surfaces, light emission surfaces (collectively called ‘lighting surfaces’) of said optical system, and said surfaces faces said vacuum zone. More specifically, it relates to an optical properties restoration apparatus and a method to use it, and such optical properties restoration apparatus is used for improving the optical property of a variety of optical systems provided outside of a light transmitting window of a variety of optical apparatus that produce combination effects of light transmission, refraction, reflection, spectrum generation and interference etc. by using high photon energy light, such as conventional ultraviolet light or vacuum ultraviolet light.
Furthermore, this invention relates to optical systems in a variety of optical apparatus and a method to use it. The optical systems improve the optical properties inside of a light transmitting window of a variety of optical apparatus that produce combination effects of light transmission, refraction, reflection, spectrum generation and interference etc. and the optical systems are provided on the light path of high photon energy light, such as conventional ultraviolet light or vacuum ultraviolet light. More specifically, the invention can be applied to the optical systems equipped with lenses, windows, etalons, prisms, reticles, and reflecting mirrors, etc., and high photon energy lamps equipped with such optical systems. Further, the invention can be applied to not only optical measurement equipments such as spectrometers, fluorescent light meters, interference meters, diffraction meters, but also to a variety of optical equipments that incorporate standard light sources for vacuum ultraviolet light, light sources for exciting chemical reactions, printing plate and photographic applications, and various light sources for experimental applications.
2. Description of the Related Art
FIG. 14 will be used to explain the component parts and operation of a microwave excited hydrogen ultraviolet lamp as an example of a conventional optical output apparatus to which this invention may be applied. This apparatus is described in Non-Patent Publication 1, which is “written by James A, R. Samson, Techniques of Vacuum Ultraviolet Spectroscopy, Pied Publications, Lincoln, Nebr., 1967, P159, Figure 5.56”. The microwave oscillator 4 has a sealed tube shaped component which is provided with both ends made of an identical electrical conductive material. The inner diameter and the length of the tube are determined by the frequency of the used microwave, and the electromagnetic field distribution to be excited in the microwave oscillator.
Microwave oscillator tuner 18 is a tube shaped component that is an essential component of the microwave oscillator that allows the adjustment of the microwave electromagnetic field distribution of the microwave oscillator, and its inside diameter is such that it envelopes discharge tube 1. Further, it is inserted concentrically with end surface of microwave oscillator 4 along their bore axes, and its structure is such that it may slide in the axial direction as it maintains its role as an electrical guide for the microwave oscillator 4. Like microwave oscillator 4, the material used to form tuner 18 is copper or brass. The function of adjusting the microwave electromagnetic field distribution by said tuner 18 is performed by adjusting its insertion depth while generating discharge plasma 7 to thereby put the microwave concentration 6 into the desired generation position.
Further, discharge tube 1 is installed in a manner such that it passes through both end surfaces of microwave oscillator 4. Although it is generally true that greatest electrical field is generated along the bore axis of discharge tube 1, which lies along the bore axis of microwave oscillator 4, this is not always the case. The cross-sectional shape of discharge tube 1 is round, but it could equally well be square, etc.
Discharge tube 1 functions as the vacuum boundary, the flow path for the discharge gas, and the space in which the discharge plasma is generated. In the example illustrated in FIG. 14, in order to limit the space in which the discharge plasma is generated, the inside tube of the conductor has been extended along discharge tube 1 from the end surface of microwave oscillator 4 toward the inside of the microwave oscillator. Accordingly, discharge plasma 7 is generated in the space between the end of microwave oscillator tuner 18 and the end of the foregoing inside tube.
Microwave oscillator 4 is connected with microwave supply connector 5 that delivers the microwaves. Here, the shape of the connector is coaxial, but it could also be of the waveguide type. Either coaxial cable or coaxial pipe may be used as the transmission feed path to the coaxial connector.
Flange 17 is attached via O-ring 13 to hold the lamp in place, on the end of discharge tube 1 at the microwave oscillator tuner 18 side. There is an opening at the center of the foregoing Flange 17, which has an inside diameter corresponding to that of discharge tube 1, which thereby allows the extraction of the light emitted from discharge plasma 7 in the axial direction of discharge tube 1.
Light transmitting window 8 that is mounted in the opening of the foregoing Flange 17 serves two functions. One is as the vacuum boundary inside of discharge tube 1 to the atmosphere. The second is to allow the extraction of the light emitted from discharge plasma 7 to outside of the vacuum. The foregoing microwave oscillator is described in detail in Non-Patent Publication 2 which is “written by E. L. Ginzton, Microwave Measurements, McGraw-Hill, New York, 1957”.
Discharge lamps having the above described constitution experience the problems described below, but before describing them, definitions of the terms used will be specified.
The vacuum ultraviolet range is the wavelength range of 0.2 to 200 nm. Light inside of that range will be termed ultraviolet light or vacuum ultraviolet light. The conventional wavelength for ultraviolet light is 200 to 380 nm (see Dictionary of Physics, published by Baifukan, and the Rika Nenpyo published by the National Observatory of Japan).
In FIG. 14, to distinguish the surfaces of the light transmitting window, the surface facing plasma discharge 7 shall be called inner surface 10, while the surface on the other side shall be called outside outer surface 11.
Problems in the Degradation of Optical Properties Outside of the Light Transmitting Window
In FIG. 14, when light generated by discharge plasma 7, especially ultraviolet light and vacuum ultraviolet light, it irradiates through inner surface 10 of transmitting window 8, passes through transmitting window 8, and passes from outside outer surface 11 of light transmitting window 8.
Here, when ultraviolet or vacuum ultraviolet light is emitted in the atmosphere such light causes the dramatic absorption of oxygen, carbon dioxide, water vapor and the like, so normally, as shown in FIG. 14, there is a mechanism on the left side of Flange 17 (to wit, outside of light transmitting window 8) which helps maintain the vacuum. The zone in which this vacuum is maintained shall be called “vacuum zone” below.
Normally, any one of a variety of vacuum pumps may be used as the mechanism to maintain the vacuum. Although there are a variety of dry pumps that are oil free (which give off almost no organic gases) that are suitable for use, rotary type pumps are the most common. Therefore, vacuum zone 14 contains organic gas from the vapor pressure of the oil used in the pump.
Further, stainless steel or aluminum metal parts, or rubber sealing parts such as O-rings are also contained within vacuum zone 14, and depending upon the application, vacuum zone 14 may contain optical elements such as samples, lenses, diffraction elements, mirrors, filters, transmitting windows, stages or other positioning elements, etc. Ideally, all of the materials in contact with the above described vacuum zone 14, to wit, the stainless steel containers, aluminum containers, O-rings and other sealing materials, optical elements, work samples, position adjustment mechanisms and the like, should be oil free (meaning that they should themselves give off almost no organic gases).
It is especially the case in the semiconductor industry that as the processing size (the width of the circuit lines) becomes finer and finer, the wavelength of the light used to make the exposure pattern for the circuit has reached the vacuum ultraviolet light range. For example the wavelength of the fluorinated argon excimer laser used as the light source for such applications is 193 nm (which when converted to energy is 6.4 eV), but in recent years, development has proceeded on laser stepper apparatus that generate wavelengths of 157 nm.
However, in actual practice, it is difficult to avoid the emission of organic gases inside vacuum zone 14, which can arise from various factors such as lubricating oil that is used for the mechanical drive structure, contamination of the work sample, out gassing from the O-rings, out gassing from plastic parts, inadequate degreasing or cleaning of the parts, or contamination introduced by human error. Thus, in actual applications, the presence of organic gases inside the foregoing vacuum zone must be considered.
Organic gases inside vacuum zone 14 have a certain probability of being adsorbed onto outside outer surface 11 of light transmitting window 8. This adsorption probability varies according to the material comprising the light transmitting window 8 and the type of organic gases, but the appearance of the adsorption phenomena itself is unavoidable.
When organic gases are adsorbed onto outer surface 11, at the same time, ultraviolet light, especially vacuum ultraviolet light, generated by the plasma irradiates these organic gases, which causes the direct excitation of the organic gas molecules to put them in an active state. This produces reactions, which draw the hydrogen, a component element of the organic gases in a dehydrogenation reaction, and finally, the adsorbed organic gas is converted into carbon (graphite). When this state is reached, it is no longer a gas but a solid, which affixes itself to and accumulates on outer surface 11 of light transmitting window 8. The carbon accumulation then adsorbs new organic gases, and their irradiation by ultraviolet light, especially vacuum ultraviolet light, converts them to carbon as well, which causes the buildup to proceed. As this process continues, outer surface 11 of light transmitting window 8 becomes covered by a carbon film. Since carbon is black, it absorbs light of various wavelengths, and as the accumulation of the carbon on outer surface 11 continues, the transmission rate through light transmitting window 8 gradually diminishes.
Here, for simplicity of explanation, it was assumed that the organic gases are hydrocarbon gases and that a dehydrogenation reaction resulted in their conversion to graphite, but in actual practice, the organic gases may include other-than-hydrocarbon elements such as oxygen, nitrogen, iodine, fluorine, chlorine, etc., and such organic gases can be adsorbed onto outer surface 11 of light transmitting window 8 just as can hydrocarbon gases, and then, through the action of ultraviolet light, especially vacuum ultraviolet light, are converted and nongaseous components are left as residuals. Thus, strictly speaking the buildup is not graphite, but it is an amorphous solid having carbon as its primary component. For purposes of describing this invention, this solid primarily comprised of carbon shall be termed “carbon.”
The phenomenon of carbon buildup requires the adsorption of organic gases and their irradiation by ultraviolet light, especially vacuum ultraviolet light. As the accumulation of carbon proceeds, the intensity of the ultraviolet light, especially vacuum ultraviolet light, that is emitted from outer surface 11 where carbon has accumulated is significantly diminished. Carbon buildup will continue until all of the light intensity is sapped. At that time, new dehydrogenation reactions cannot take place and the accumulation of the carbon film stops. Accordingly, this process is not one where a carbon film can grow without limits, but the phenomenon ceases once a limit film thickness has been reached.
Normally, the phenomenon of carbon buildup on outer surface 11 of the foregoing light transmitting window 8 does not proceed rapidly. The problem is one of diminishing transmission through the light transmitting window 8 over a long period of time. In spectrographic applications, when the quantity of light from the light source diminishes, it creates drift which affects the accuracy of the measurements, and in applications involving surface treatments by ultraviolet light irradiation, problems arise due to inadequate processing caused by diminished irradiation intensity.
One means of addressing this problem of the carbon buildup phenomenon is to strive for an oil-free vacuum zone 14, however, once an organic substance has contaminated vacuum zone 14, the cleanup process is extremely difficult. Accordingly, the conventional countermeasure to diminished transmission rates due to carbon accumulations on outer surface 11 of light transmitting window 8 involve using cleansers or polishing to remove the carbon to restore light transmitting window 8 to its original state, or replacing light transmitting window 8 entirely.
In the prior art, the decline in the light transmission rate of light transmitting window 8, to wit, its degradation, was the determining factor in lamp longevity. Lamps that had reached their longevity, had their light transmitting windows 8 cleaned or replaced, which required breaking the vacuum in vacuum zone 14 or in the lamp. This operation required several hours time during which the lamp could not be used.
Next, conventional countermeasures in response to degradation of the transmitting window due to carbon buildup will be described.
The technology disclosed in Japan Patent Application Publication No. 2001-319618 (Patent Publication 1) will be described below.
In this example, the light source in question was a hydrogen lamp. When the hydrogen is introduced into the discharge tube, a halogen is also sealed within as a means to increase the lamp's longevity. The halogen sealed therein is in the form of an organic halogen compound. This means that an organic halogen substance has been introduced into the discharge area. Then, when the lamp is in operation, the organic material decomposes and causes a film of organic material, primarily carbon, to adhere onto the inner wall of the discharge tube. This inner wall functions as the light transmitting window, and the adhesion of material on its walls invites a reduction in the quantity of light generated. As a countermeasure, the above-cited Patent Publication 1 proposed a pre-shipment treatment of the lamps that could forcibly cause a carbon film to adhere to the region that functioned as the light transmitting window where carbon was envisioned as building up during lamp operation, and then, during the normal operation of the lamp no additional carbon buildup would occur. This technology considered that there were finite limits to the generation of organic material and that this countermeasure would effectively create an environment where no new buildup would occur during the operation of the lamp.
However, as described above for vacuum zone 14, if the apparatus involved was one that required repeatedly opening to the atmosphere or vacuum release (such as in a spectrographic application where samples have to be replaced, in applications where optic elements have to be adjusted, or where work pieces need to be exchanged during surface treatments, etc.), even if the spec calls for no organic contamination to be introduced during the assembly and adjustment processes, not introducing such contamination is rare in actual practice and hence, it would be impossible to avoid degradation of light transmitting window 8
Further Japan Patent Application Publication No. 2001-293442 (Patent Publication 2) relates to a cleansing method to remove adsorbed organic materials from the surfaces of optical elements by means of a method that minimally includes: (1) a process to cleanse the optical elements with an organic solvent, (2) a process to irradiate the optical elements with ultraviolet light in the presence of oxygen, and (3) a process to heat and cleanse said optical elements. Not only does this disclosure have an objective that differs from removing accumulated carbon films from surfaces, but it further does not resolve the problem of cleaning optical elements such as the light transmitting window when there is a need to break the vacuum.
Further, Japan Patent Application Publication No. 2001-219429 (Patent Publication 3) discloses technology that is similar to that of the present invention. Its objective is to improve the treatment precision and treatment efficiency of cleansing, etc., and is characterized in that the surfaces of substrates including glass substrates, synthetic resin substrates, ceramic substrates, metal substrates, and composite substrates comprised of 1 or a plurality of the foregoing substrates are wetted on the surface inside of a heated gas atmosphere containing water vapor and subsequently, the substrate is irradiated with ultraviolet light in a mixed atmosphere of heated inactive gas and water vapor, which is at a lower concentration than was present in the wetting atmosphere, which thereby serves to dissolve organic substances adhering to the surface of said substrate, and moreover, reduction active seeds [H—] and oxidizing active seeds [—OH] are generated, and these active seeds [H—] and active seeds [—OH] react with the products of decomposition of the organic material.
The objective of said prior technology is not to remove the carbon film from surfaces, but rather, it aims to dissolve the organic adherents to substrate surfaces by reducing them to smaller molecules, especially with regard to irradiating the substrate surface with ultraviolet light, and cleansing and etching processes, which use the ultraviolet light irradiation as a means of substrate treatment. Not only does the objective differ from the present invention, but since the substrate surface must be in a saturated condition, it is premised on the water being a liquid under the reaction conditions. As a result, the method can only be used in an environment of near normal atmospheric pressure—it cannot perform optical element cleansing of light transmitting windows and the like under vacuum conditions, and it does nothing to resolve the problems that develop when it is necessary to break the vacuum.
Although the explanation up to this point has been limited to the phenomenon that occurs on the outer surface 11 of light transmitting window 8, this type of carbon buildup phenomenon is not confined to only outer surface 11 of light transmitting window 8. It is generally the case that the phenomenon of carbon buildup occurs on the surfaces of objects located in vacuum zone 14 that are irradiated by ultraviolet light, especially vacuum ultraviolet light, that is emitted from the light transmitting window 8. This phenomenon is unavoidable so long as the conditions of the presence of organic gases and ultraviolet light, especially vacuum ultraviolet light, coexist. The “objects” referred to in the foregoing explanation include the mirrors that switch the light path in spectroscopic applications, filters, lenses for focusing light and diffraction elements used in spectroscopic applications, lenses used for focusing light, and various filters used in surface treatment applications, in other words, any of a variety of optical elements. Hereinafter, any of such objects will be referred to collectively as “optical elements.” When carbon accumulates on these optical elements it causes serious problems by reducing their light transmission and light reflectivity. In actual practice, it lowers or causes the total loss of function of apparatus used in vacuum zone 14.
Formerly, to counter such diminishment of the light transmission and reflection, such optical elements had to be replaced with new ones, but this approach leads to high maintenance costs and keeps the apparatus out of service for the time required for maintenance.
The problem with lamp longevity due to the deterioration of light transmitting window 8 is not confined to the microwave-excited hydrogen ultraviolet lamps that were described in the foregoing example, similar problems exist for a wide variety of lamps such as those using He, Ne, Ar, Kr, Xe, O2, N2, D2 (deuterium molecules), Hg, etc.; lamps using high frequency discharge, arc discharge, glow discharge, inductive barrier discharge, or flash discharge in their discharge mode; or in halogen lamps or carbon lamps that heat a filament using electrical current as their means of light generation.
Problems in the Degradation of Optical Properties Inside of the Optical Systems
The problem of lowing the light transmitting property from short wavelength ultraviolet light is not limited to the degradation of optical properties outside of the light transmitting window, but also the degradation of optical properties inside of the light transmitting window.
In recent years, in order to obtain better light transmitting properties from short wavelength ultraviolet light, SiO2 has been developed for use as the foregoing light transmitting windows.
Further, a problem with mercury vapor lamps is more serious comparing to the previous time, that the quartz glass used in them loses its clarity. The quartz glass in a mercury vapor lamp functions as the vacuum boundary to the outside for the inside of the lamp, and it also functions to transmit the ultra violet light that is generated from the luminescence of the mercury, but the phenomenon losing the clarity degrades its light transmission properties and is a factor in the determination of lamp longevity.
In Japanese Patent Application Publication No. Hei 5-325893 (Patent Publication 4), for example, a countermeasure for losing the clarity was proposed as using a light emitting tube as the metal vapor electrical discharge arc tube, which employed a roughened inside surface of the glass bulb exhibiting a surface grain size of under 1 micron. So doing would impede the crystallization (losing the clarity) of the arc tube even after it had been operating for long periods of time, and thereby impede the decline in light flux (illumination sustenance rate) to make it possible to maintain bright pictures and high quality displays over a long period of time for projection type displays.
This technology was applied to quartz glass or high silicate glass used in arc tubes, and although it would be possible to apply it to conventional ultraviolet light applications in the 250-360 nm wavelength range, the transmission rate of quartz glass for vacuum ultraviolet light of a wavelength of 190 nm diminishes substantially.
Further, Japanese Patent Application Publication No. Hei 3-77258 (Patent Publication 6) discloses technology for 254 nm ultraviolet light constant pressure mercury vapor lamps, wherein the inside surface of synthetic quartz glass is coated with a 1 to 3% solution of metal oxide particles having an average particle diameter under 100 μm, which in the examples consisted of metal oxide having an average particle diameter of 20 μm.
Further, Japanese Patent Application Publication No. Hei 8-212976 (Patent Publication 7) discloses technology for a discharge lamp using an arc tube comprised of a quartz glass tube with mercury sealed within and electrodes sealed on each end, that employed a thin film coating of Al2O3, etc. on the inside of the tube, wherein the thin film is thicker on the inside surface of the foregoing arc tube near the center of the tube than it is in the other areas; specifically, the thick film area on the inside surface of the foregoing arc tube is ⅓ to ½ the length of the effective light emission length, which is the distance between the electrodes on either end of the arc tube, and is such that the film thickness in the aforementioned thick film area ranges from 0.2 μm to 0.3 μm, while the film thickness in the other areas ranges from 0.1 μm to 0.15 μm.
However, this prior art technology related to quartz glass, especially to that used in low pressure mercury discharge lamps, and it regulated the thickness of the protective film where the mercury atoms existed as a means to deal with the problem of the mercury being deposited upon the inside wall of the arc tube to thereby lower the transmission rate of light through the quartz glass and cause the blackening of the discharge lamp, which further diminishes its irradiation efficiency.
Furthermore, the lower limit for SiO2 delivering good light transmission rates is about the 200 nm level; light transmission drops dramatically with the shorter wavelength vacuum ultraviolet light that exhibits wavelengths lower than 200 nm. Furthermore, with very short wavelengths of vacuum ultraviolet light in the 150 nm vicinity such as used with high energy fluorine lasers, not only does the foregoing light transmission rate decline, but the material cannot stand up to the application and the losing the clarity occurs.
Also, considering the fact that with synthetic silica glass, there is a significant decrease in the transmission rate in the ultraviolet light range through window materials through which irradiated lamp light is transmitted, Japanese Patent Application Publication No Hei 8-315771 (Patent Publication 5) discloses fluorine doping technology for synthetic silica glass that aims at improving operational longevity.
However, using fluorine compounds to dope the silica glass base stock only allows about a 50% transmission range in the 160-190 nm wavelength ranges, and it cannot stand up to use in the lower wavelengths of vacuum ultraviolet light.
Accordingly, alkali halide materials, such as CaF2, LiF, MgF2, etc., have generally been used for light transmitting window stock when vacuum ultraviolet range ultraviolet light had to be transmitted.
A suitable example from the prior art is the aforementioned microwave-excited hydrogen ultraviolet lamp, which generated vacuum ultraviolet light at a wavelength of 122 nm. The only known materials that could be used for light transmitting windows were CaF2, LiF, and MgF2, and since LiF and CaF2 exhibited dramatically lower light transmission from their color center, MgF2 was most commonly used. However, there has been no disclosure of any report that dealt with countermeasures of losing the clarity for MgF2.
To wit, when the magnesium fluoride is used as the material for light transmitting windows, such windows exhibit a poorer longevity than other window materials, and compared with lamps that use other window materials, lamp longevity itself is only about half or less.
When using light with a higher photon energy than the absorption wavelength for the material used in light transmitting window 8, especially light in the vacuum ultraviolet range, when the light from the discharge plasma is irradiated upon light transmitting window 8, said window 8 develops a defect, a so-called color center is produced that lowers its light transmission rate. This phenomenon is also common to CaF2, LiF, MgF2 and other alkali halide materials, and is caused by the slight shift of fluorine atoms from their correct position within the lattice.
Further, the aforementioned conventional technology all addressed problems associated with synthetic quartz, especially synthetic quartz optical systems that used conventional wavelength ultraviolet light as the light source. There have been no proposals for practical technology that was effective in preventing the diminishment of the light transmission rate through MgF2, which is the material used in light transmitting windows for the 122 nm wavelength vacuum ultraviolet light generated by microwave-excited hydrogen ultraviolet lamps.
Due to this situation, when the transmission rate declines, the only way to deal with it is to replace the light transmitting window. In this prior art, the degradation of light transmitting window 8 as described above was the determining factor in lamp longevity. In the prior art, once the lifespan of the lamp's light transmitting window 8 was up, it would have to be replaced with a new light transmitting window to restore the light emission intensity of the lamp. The replacement of light transmitting window 8 requires the breaking of the lamp's vacuum and several hours of labor, during which time, the lamp cannot be used. Further, during the replacement cycle, the output intensity from the light source is constantly changing. Each time the transmitting window is replaced, it requires calibration operations for the light intensity. Thus, it is difficult to use such lamps in applications that require long-term monitoring, such as employed in environmental measurements.